The invention relates to an oxidized retreated carbon black within the MCC area which may be used advantageously as a carbon black pigment for coloring high jet black lacquers.
Such lacquers are used, for example, in the automobile industry, for optical instruments, for solar collectors and the like. In these black lacquers, carbon black is preferred as pigment over all others, because carbon black allows coloring with the highest degree of blackness and comparison to organic pigments, has practically an unlimited resistance to light and heat.
In such high jet lacquer systems, of which the highest requirements are made in regard to degree of blackness, surface and gloss, usually carbon blacks of the HCC area (high color channel black) or the HCF area (high color furnace black) are used, that is to say, carbon blacks are used which are produced according to the channel process or according to the furnace process.
These processes are described in detail in Ullmanns Enzylkopadie der Technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 4th Edition, Vol. 14, p. 636 ff. which is relied on herein for the purpose of including a description of conventional methods of producing carbon blacks.
In the case of the channel process which starts out from natural gas as a raw material, carbon black is produced in a large number of small flames which burn against cooled iron rails (channels) on which the carbon black is then deposited. It is important that the carbon black in this channel process is formed in a gaseous atmosphere which has an excess of atmospheric oxygen. As a result of that, oxygen containing groups with an acid character (pH value about 3) develop on the surface of the carbon black. Because of the uneconomical aspect of this process, in the case of which only 3-6% of the carbon is obtained in the form of carbon black, the channel process is no longer used for practical purposes.
The gas carbon black (gas black) process operates essentially similarly. In that case, instead of natural gas, a carrier gas (coke oven gas, city gas, hydrogen, etc.) loaded with oil vapors is conveyed to the burners. The flames strike against cooled rotating rollers on which the carbon black deposits. In the case of this process too, the formation of carbon black takes place in an atmosphere of excess oxygen and therefore, the carbon black obtained thereby shows an acid reaction in water.
Because of the great similarity to the carbon blacks obtained according to the channel process, carbon blacks produced according to the gas process are also classified with channel blacks.
Since the yields of carbon black in that case are approximately 10 times higher than in the case of the channel process, the gas black process may compete with the furnace process.
The furnace process operates in a closed system:
In a ceramically coated furnace, a hot flame is produced from gas and air into which the carbon black raw material in the form of highly aromatic oils is atomized. The volume of atmospheric oxygen related to the quantities of gas and oil used is present in a deficient amount.
After the formation of the carbon black, water is sprayed into the reactor and the mixture of gas and carbon black is cooled to about 900.degree. C.
The gas atmosphere, different as compared to the previously mentioned processes, leads to the fact that furnace blacks have fewer surface oxides and show neutral to weakly alkaline reactions (pH value: 7-10).
Since the surface groups of carbon blacks interact action with the molecules of the binders, the surface chemistry is of great importance in respect of the ability of a carbon black to be dispersed in a binder for lacquers and this among other things is also decisive for the color value of a lacquer.
Carbon blacks with acid surface groupings are particularly suitable for the overwhelming part of lacquer binders.
Acidic surface oxides may be generated on the furnace blacks or else may be multiplied in case of the channel black types, whenever these carbon blacks are subjected to an oxidative after-treatment. An oxidation may be executed with NO.sub.2 -air mixtures in the fluidized bed or else with nitric acid in the fluid phase.
The degree of oxidation of a carbon black may be determined subsequently by the determination of the volatiles.
A carbon black according to DIN 53 552 in a platinum crucible with a well closing lid which has a 2 mm hole (crucible according to DIN 51 720), is annealed for 7 minutes at 950.degree. C. in a muffle furnace.
The loss in weight during annealing is stated in percent of the weighed sample and represents the "volatiles" which are considered as a measure of the quantity of surface oxides.
Generally speaking, the carbon blacks are classified according to the size of their primary particles. This particle size may be determined with the electron microscope.
Thereby, the diameters of a large number of particles are determined and the arithmetic mean is calculated on the basis of several electron microscope pictures of the carbon black sample with the help of the semi-automatic particle size analyzer TGZ 3 according to Endter and Gebauer (Carl Zeiss, Oberkochen). The method was developed by F. Endter and H. Gebauer, as described in Degussa, Optic 13, 97-101 (1966) which is relied on herein to show this method. Recent measures in the case of which the object space of the electron microscope is cooled, lead to clearly lower values of the particle size.
The particle diameters stated here are based on measurements without object space cooling, in order to be comparable to data published hitherto.
______________________________________ Classification of the Carbon Blacks: Primary Particle Channel Blacks Furnace Diameters Gas Blacks Blacks nm ______________________________________ HCC HCF &lt;15 MCC RCF 15-20 RCC RCF &gt;20 ______________________________________ HC = high color MC = medium color RC = regular color C: channel blacks F: furnace blacks
The primary particle size is of great importance because it is responsible for a large series of analytical data and for industrial application data and, not in the least, it is also responsible for the profitability of a carbon black; for a better characterization of a carbon black, various tests procedures are in use:
In view of the use of a carbon black for black lacquers, the determination of the color depth of a carbon black is important.
If carbon blacks of different particle size are compared, then it turns out that types of carbon blacks with coarser particle sizes tend more towards gray, i.e. less color depth than finely dispersed carbon blacks with which one may achieve a greater depth of color.
A measuring method, for the nigrometer index, is described subsequently:
0.1 g of carbon black are carefully ground together with a linseed oil varnish (RAL 848 B) on a glass plate with an elastic steel spatula until a homogenous "standing" paste has formed; at the same time, the oil is added slowly from a 2 ml burette. (For more details, see under oil absorption).
Densified carbon blacks are to be comminuted dry with the spatula prior to the addition of oil.
The paste is painted thickly onto a slide and the light remission of the paste is measured right through the glass with a nigrometer. The recording takes place as the nigrometer index, whereby small numbers state a high degree and large numbers a low degree of depth of color.
The depth of color may also be determined very well by a visual comparison of the paste with standard pastes of known nigrometer indices. For this purpose, the sample and the comparative paste are painted side by side thickly onto a slide and the depth of color is evaluated right through the glass in a very bright light (Leitz lamp).
This process produces particularly good results whenever the sample paste is spread between the paste of a brighter and of a darker standard carbon black. In order to achieve reproducible results, it is assumed that always clear and clean glasses are used.
The connection between particle size and nigrometer index becomes clear from the following listing:
TABLE 1 ______________________________________ Types of Particle Size Carbon Blacks (Diameter Depth of Color (Class) in nm) (Nigrometer Index) ______________________________________ HCC 13 63 HCC 15 68 MCC 17 71 MCC 20 76 RCC 25 80 ______________________________________
Another, important data for a characterization of carbon black is the specific surface area. It is related to the particle size. In the case of carbon black which have a surface free of scars and pores, the specific surface area is the higher the more finely dispersed a carbon black is. The specific surface may be determined in accordance with BET. (Brunauer, Emmett and Teller).
The surface area of a solid body may be calculated from the N.sub.2 adsorption isotherm recorded at the boiling temperature of the liquid nitrogen.
By evaluation of the adsorption curve in the relative pressure area between p/p.sub.o .about.0.05 and p/p.sub.o .about.0.2, the volume VM is obtained, which according to the theory of Brunauer, Emmett and Teller represents the quantity of nitrogen needed for a monomolecular covering.
16.2 A.sup.2 are made the basis for the cross sectional area need of a nitrogen molecule. From this one can calculate the surface area of the sample accessible to the N.sub.2 molecules. For details, see for example, Brunauer, Emmett and Teller: J.A.C.S. 60, (1938), 309. The BET surface is stated in m.sup.2 /g.
On the other hand, the purely geometric surface area of a carbon black may also be calculated from electron microscopic determinations of particle sizes.
Deviations between the surface area values determined by adsorption and those calculated from the particle size are conditional on the porosity of the carbon blacks.
The oil requirement offers a help of orientation for the consumer in the case of the selection of various carbon blacks for use in lacquers and dyes. The oil absorption however is influenced by the particle size but also by the surface chemistry and the "structure" of the carbon black. (By "structure" one understands the degree of coalescence of primary particles to larger chain or grapeshaped aggregates.)
The oil absorption (stiff paste) is determined as follows:
0.5 g of carbon black are ground carefully with linseed oil varnish (RAL 848 B) with an elastic steel spatula on a glass plate until a homogenous "stiff paste" has formed.
Thereby, the oil is added slowly from a 2 ml burette. The terminal point of the addition of oil is recognized from the fact that in case of lifting the spatula from the paste, one can draw coneshaped peaks. These peaks do not collapse immediately but only as a result of shock (for example, a small tap on the glass plate).
This is the reason for the term "stiff" paste or also flow point. The oil absorption is stated according to a recalculation of the consumed ml of linseed oil in gram in percent by weight related to the carbon black. The accuracy of the measurement is .+-.20%.
The values for the oil absorption are generally the higher, the more finely dispersed a carbon black is. Whenever a carbon black powder is densified or changed into a granulate, the values for the oil requirement drop.
By the pH value of a carbon black, is understood to mean the pH value of an aqueous suspension of the carbon black. The determination is made in accordance with DIN 53 200.
For this purpose, 1 g of carbon black is weighed in a 50 ml Erlenmeyer flask, 20 ml of freshly distilled water as much as possible free of CO.sub.2 is added and is stirred vigorously with a magnetic stirrer for 2 minutes. Subsequently, the glass electrode is submerged directly into the suspension and after 1 minute the pH value is read from the measuring device. Beaded carbon black must be powdered before being weighed.
The data concerning industrial application of a carbon black are determined in connection with the production of a black lacquer (composition of the ground materials, rheology, dispersibility) and after the production of a test film of lacquer. Decisive values which may be measured for a lacquer film are depth of color, color tone, surface quality, gloss. For determining characteristics for industrial use, first of all a carbon black concentrate in the form of a ground material is produced and is further processed into two test lacquers: